This function is used to asynchronously write a certain number of bytes
of data to a stream. The function call always returns immediately. The
asynchronous operation will continue until one of the following conditions
is true:

All of the data in the supplied buffers has been written. That is,
the bytes transferred is equal to the sum of the buffer sizes.

An error occurred.

This operation is implemented in terms of zero or more calls to the stream's
async_write_some function, and is known as a composed operation.
The program must ensure that the stream performs no other write operations
(such as async_write, the stream's async_write_some function, or any other
composed operations that perform writes) until this operation completes.

The stream to which the data is to be written. The type must support
the AsyncWriteStream concept.

buffers

One or more buffers containing the data to be written. Although the
buffers object may be copied as necessary, ownership of the underlying
memory blocks is retained by the caller, which must guarantee that
they remain valid until the handler is called.

handler

The handler to be called when the write operation completes. Copies
will be made of the handler as required. The function signature of
the handler must be:

voidhandler(constboost::system::error_code&error,// Result of operation.std::size_tbytes_transferred// Number of bytes written from the// buffers. If an error occurred,// this will be less than the sum// of the buffer sizes.);

Regardless of whether the asynchronous operation completes immediately
or not, the handler will not be invoked from within this function.
Invocation of the handler will be performed in a manner equivalent
to using boost::asio::io_service::post().